In electrostatography, an image comprising an electrostatic field pattern, typically of non-uniform strength, which is usually referred to as an electrostatic latent image, is formed on an insulative surface of an electrostatographic element by any of various methods. For example, the electrostatic latent image may be formed electrophotographically, i.e., by imagewise photo-induced dissipation of the strength of portions of an initially uniform electrostatic field of uniform previously formed on a surface of an electrophotographic element comprising a photoconductive layer and an electrically conductive substrate. It may also be formed by dielectric recording, i.e., by direct electrical formation of an electrostatic field pattern on a surface of a dielectric material. Typically, the electrostatic latent image is then developed into a toner image by contacting the latent image with a developer composition containing charged toner particles. If desired, the toner image can be transferred to a final support material or receiver such as a web or sheet of paper and affixed thereto by, for example, thermal fusing at a fusing station that typically includes two rollers, at least one of which is heated. A permanent record of the original is thereby formed.
The transfer of toner images between supporting surfaces has been accomplished using either a transfer roller or belt electrode biased to a certain potential, or a coratron. In corona-induced transfer as disclosed, for example, in U.S. Pat. No. 2,836,725, the disclosure of which is incorporated herein by reference, the final support sheet is placed in direct contact with the toner image while the image is supported on the photoconductive surface. The back of the sheet, that is, the side away from the image, is subjected to a corona discharge having a polarity opposite to that carried by the toner particle, thereby causing the toner to be electrostatically transferred to the sheet. In the corotron system, electrostatically deposited charges tack a final support such as, for example, paper to the original toner support, for example, the photoconductor, at the same time creating the electrical field required to effect transfer of the toner to the paper. However, the strong attraction between the paper and the original toner support makes it mechanically difficult to separate the two supports.
A biased transfer member, for example, a roller or drum, electrically cooperates with a conductive support surface to attract electrically charged particles from the support surface towards the transfer member. Transfer of developed images from the photoconductor to the final support using a biased transfer member is well known in the art. In U.S. Pat. No. 2,807,233, the disclosure of which is incorporated herein by reference, a metal roller coated with a resilient coating having a resistivity of at least 10.sup.6 ohm-cm is used as a bias transfer member. Because of the
high resistivity of the coating, the amount of bias that can be applied to the roller is limited to relatively low operating voltages. At higher voltages, the air in or about the transfer zone begins to ionize, causing the image to be degraded during transfer. In U.S. Pat. No. 3,520,604, the disclosure of which is incorporated herein by reference, is described a transfer roller made of a conductive rubber and having a resistivity in the range of 10.sup.16 to 10.sup.11 ohm-cm. Here, in order to give the roller the resiliency required for most applications, the coating must be relatively thick. The resulting high resistivity would be expected to cause charge to build up on the surface of the roller, resulting in air ionization in the transfer region and eventual copy degradation.
Other biased transfer members have been disclosed that purport to overcome many of the electrical and image degradation problems associated with some of the previous transfer techniques. U.S. Pat. No. 3,702,482, for example, the disclosure of which is incorporated herein by reference, describes a transfer member having an outer coating with electrical resistivity intended to minimize ionization of the surrounding atmosphere when the transfer member is placed in electrical cooperation with a conductive support surface. In U.S. Pat. No. 3,781,105, the disclosure of which is incorporated herein by reference, is described a similar transfer member employed in conjunction with variable electrical bias means to regulate automatically the electrical field levels at various points on the transfer member during the transfer operation, with the object of providing constant current control.
In duplex electrophotographic processing, in which a fused toner image is formed on both sides of a receiver sheet, release oil is applied at the fusing station to the first imaged side of the receiver sheet. When the sheet is turned over for imaging of the reverse side, the oiled first side comes in contact with the transfer member. A portion of the oil from the first side can adhere to the surface of the transfer member and from there be transferred to the photoconductor in an interframe area between receiver sheets. Toner particles that accumulate on the oil-contaminated area of the photoconductor can be transferred to subsequent imaged receiver sheets, resulting in high background density and degraded images.
The application of release oil at the fusing station can be effected by various means such as a roller, a pad, a wick, and the like. Clearly, it is desirable that toner fuser release oil be delivered during copying at a controlled, substantially constant rate. This desirable steady state of oil delivery can, however, be disrupted by various events such as, for example, installation of a new pad or wick or occurrence of a rest period between copying runs. Such events can lead to excess oil being delivered to the fusing station before equilibrium is reestablished, which aggravates the problem of oil contamination of the photoconductor. This problem is substantially mitigated by the toner transfer member of the present invention.